During the Battle of Marineford, when Akainu killed Ace, Garp was going to attack Akainu but Sengoku stopped him. At that point Garp said to Sengoku to not let him go otherwise he(Garp) will kill Akainu. So do you think Garp would really had killed Akainu? Was Garp really that much powerful at that time that he might had killed Akainu, possibly the strongest admiral?
Did you Know that Eiichiro Oda asked several famous Mangakas to draw Luffy and his crew in their own styles?